In recent years, as the capabilities of color copy machines have improved, the counterfeiting of bank notes, negotiable securities and similar documents has become more frequent. This is why these types of color copy machines now have a built-in circuit to generate counterfeit probability data indicating the probability that an attempt at counterfeiting is occurring (hereafter referred to as a "counterfeit detecting device"). These data are generated by comparing a characteristic pattern found in the image which is read (for example, a characteristic mark added to a non-reproducible document or a portion of a pattern which already occurs on the document) to a set of reference patterns. When the counterfeit probability data generated by the device suggest that a counterfeiting attempt is probably occurring, the print output can be controlled in an appropriate way, such as by preventing the reproduction of the document in question or by copying a black screen. In this way, this sort of counterfeiting can be effectively prevented.
Heretofore, the patterns used as references for comparison in this type of device are identical to patterns found on the non-reproducible document. For example, if the pattern on the non-reproducible document is the numeral "0", the reference pattern used for comparison would also be a "0".
However, with this type of prior art device, the fact that the numeral "0" is used as the reference pattern to judge when a counterfeiting attempt is occurring is readily discernable. If the pattern is tampered with until it looked like a "9", a person would not notice anything unusual about the document and a machine would recognize the nine as a completely different numeral. As a result, it would not recognize that the document is non-reproducible and would permit it to be counterfeited.
It might be conceivable to address this problem by broadening the range of concurrence between the pattern on the document and each of the reference patterns. However, this will cause some ordinary documents to be misidentified as non-reproducible, so it is not a convenient solution.
This invention is developed in consideration of the aforementioned problems.